This invention relates to a cruise control apparatus for an automotive engine. More specifically, it relates to a cruise control apparatus of the type which maintains the speed of an automobile by controlling the throttle valve of the carburetor.
A cruise control apparatus is a device which enables a driver to maintain the speed of an automobile at a desired level without his having to use the accelerator pedal. One type of cruise control apparatus maintains the vehicle speed by controlling the degree of opening of the throttle valve of the engine. In this type of apparatus, the throttle valve is adjusted by a motorized or vacuum-operated actuator through a wire cable and a link mechanism, and the actuator is controlled based on a speed-dependent electrical signal from the speedometer.
In a cruise control apparatus with a motorized actuator, the actuator comprises a motor having a rotating output shaft on which a pulley is mounted. A wire cable which is wrapped, around the pulley is connected with the throttle valve of the engine through a link mechanism. The rotation of the output shaft causes the wire cable to open or close the throttle valve, depending on the direction of rotation. The speed of rotation of the output shaft of the actuator is dependent on the electrical power supply for the engine, which is substantially constant over the entire range of engine operation. Therefore, the speed of movement of the throttle valve is substantially constant, regardless of the speed of the vehicle. A graph of the speed of opening of a throttle valve as a function of vehicle speed for a conventional cruise control apparatus with a motorized actuator is therefore a straight horizontal line, as shown by curve A of FIG. 5.
On the other hand, a vacuum-type actuator has a diaphragm which is caused to reciprocate by vacuum from the intake manifold. The diaphragm is connected to the throttle valve by a tension member such as a cable and a link mechanism. The speed of movement of the diaphragm, and thus the speed of opening of the throttle valve, depends on the magnitude of the vacuum in the intake manifold, the orifice diameter of control valves for the actuator, and a number of other factors, but in general, it increases nonlinearly as the intake manifold vacuum increases.
While a constant speed of throttle valve opening over the entire range of vehicle speed, such as is provided by a cruise control apparatus with a motorized actuator, is satisfactory for a commercial vehicle with a heavy load, it is not suitable for sports cars or passenger cars. In order to obtain the responsiveness desired of a sports car, the speed of throttle opening should increase linearly with increasing vehicle speed up to a fairly high vehicle speed. In a higher speed range, the speed of throttle opening should gradually and nonlinearly increase to a maximum, while in a still higher speed range, it should gradually and nonlinearly decrease from the maximum. The preferable operating characteristics of a cruise control apparatus for a sports car are illustrated by curve C of FIG. 5, which plots the speed of throttle valve opening as a function of vehicle speed. The operating characteristics for a passenger car should be similar, as shown by curve B of FIG. 5. However, as a passenger car requires less responsiveness than a sports car, the region of linearly increasing throttle valve opening speed is narrower than for a sports car, and the maximum speed of throttle valve opening is less.
Unfortunately, it is extremely difficult to obtain the operating characteristics illustrated by curves B and C using a conventional cruise control apparatus having a motorized or vacuum-type actuator, and the responsiveness of sports car or passenger car with a conventional cruise control apparatus is unsatisfactory.